The researchers, whose analysis of the platypus genome was published Thursday in the journal Nature, said it could help explain how mammals, including humans, evolved from reptiles millions of years ago.
The platypus is classed as a mammal because it has fur and feeds its young with milk. It flaps a beaver-like tail. But it also has bird and reptile features - a duck-like bill and webbed feet, and lives mostly underwater. Males have venom-filled spurs on their heels.
"At first glance, the platypus appears as if it was the result of an evolutionary accident," said Francis S. Collins, director of the U.S. National Human Genome Research Institute, which funded the study.
"But as weird as this animal looks, its genome sequence is priceless for understanding how mammalian biological processes evolved," Collins said in a statement.
The research showed the animal's multifaceted features are reflected in its DNA with a mix of genes that crosses different classifications of animals, said Jenny Graves, an Australian National University genomics expert who co-wrote the paper.
"What we found was the genome, just like the animal, is an amazing amalgam of reptilian and mammal characteristics with quite a few unique platypus characteristics as well," she told the Australian Broadcasting Corp.
Scientists believe all mammals evolved from reptiles, and the animals that became platypuses and those that became humans shared an evolutionary path until about 165 million years ago when the platypus branched off. Unlike other evolving mammals, the platypus retained characteristics of snakes and lizards, including the pain-causing poison that males can use to ward off mating rivals, Graves said.
More than 100 scientists from the United States, Australia, Japan and other nations took part in the research, using DNA collected from a female platypus named Glennie.
Their work adds to the growing list of animals whose genetic makeup has been unraveled.
By comparing platypus genes to those of humans and other mammals, scientists hope to fill in gaps in knowledge about mammals' evolution and better identify certain species' specific traits.
Des Cooper, an evolutionary biologist at the University of New South Wales who did not take part in the research, said it represented a big step forward in the world's knowledge of mammals.
"Platypuses are often thought of as primitive because they lay eggs," Cooper said. "This paper demonstrates there is a mixture of characters, which they share with other mammals, and of highly specialized attributes."
Graves said the research contained some surprises, such as the conclusion that genes which determine sex in a platypus are similar to those of a bird, not a mammal. Researchers also found genes that indicate platypuses - which rely on electrosensory receptors in their bills to navigate as they rummage with closed eyes in waterways - may also be able to smell underwater.
Unique to Australia, the platypus has confounded observers for centuries. Aboriginal legend explained it as the offspring of a duck and an amorous water rat. When the British Museum received its first specimen in 1798, zoologist George Shaw was so dubious he tried to cut the pelt with scissors to make sure the bill had not been stitched on by a taxidermist.
Platypuses live in the wild along most of Australia's east coast. Their numbers are not accurately known because they are notoriously shy. Hunted for years for their pelts, they have been protected since the early 1900s and are not considered to be endangered, though scientists say their habitat is vulnerable to human development.
